Gas turbine engines like other rotating machines or turbomachines are required to operate at a range of modes of operation. Particularly there may be the need to operate a gas turbine engine on a broad range of loads. Additionally there may be the need to operate a gas turbine engine on different levels of ambient temperature. Furthermore there may be requirement to fulfill emission demands for all different modes of operation.
In the gas turbine engine typically ambient air as a fairly cool fluid may be compressed by a compressor—e.g. a compressor section of a gas turbine—and provided to a combustor in which the still substantially cool fluid (the temperature may be several hundred degree Celsius at a discharge of the compressor section but at least drastically cooler than the working conditions within the combustor of the gas turbine engine) will react together with fuel to provide a driving force for a subsequent turbine section in which a hot and pressurised fluid from the combustor will drive rotor blades of the turbine to drive again a shaft. The shaft may again drive blades of the compressor section to perform the compression of the ambient air and the excess power will drive an electric generator or a rotating load attached to the shaft of the gas turbine engine.
In the combustor or in the downstream sections past the combustor high temperatures can occur on components that are guiding the hot fluid through the gas turbine engine. The temperatures can be up to 1,500° C. or even higher. High combustion temperatures may be advantageous in respect of emissions, e.g. carbon monoxide (chemical symbol: CO) and unburned hydrocarbon emissions.
Furthermore it has to be noted, even though this may degrade the efficiency of the compressor, it is known that not all of the compressed air may be used for combustion. A part of the air may be used in some other different ways, like de-icing at the compressor entry or the inlet of the gas turbine engine, or turbine blade cooling or cooling of other components. Therefore, compressed air is taken from within the engine, typically in or after the compressor stage(s), depending on the application. This deviated compressed air is typically called bleed air and is typically not provided to the burners for combustion.
There may also be reasons to extract air from the compressor and guide this directly to the exhaust of the gas turbine engine. Even though this is a planned measure, this leads to a reduced overall efficiency. Furthermore, potentially extraction of air can produce an unbalance in the compressor flow that could induce compressor instabilities—like compressor surge, stall—or engine instabilities e.g. uneven engine temperature, flame flash back in burners.
It has to be noted that as a general aim typically in gas turbine design it is aimed for lean combustion at full load with supply of as little fuel as possible, particularly to reduce the expenditure for fuel. The lean combustion may typically be configured and optimised for full load operation.
It may be problematic to run the gas turbine engine in continuous low load operation as low load operation is typically far from optimal design conditions. For transient low load operation it may not be a problem to exceed emission legislation for a short time, but for continuous low load operation given emission requirements should be met.
It is an aim to provide a solution to extend the range of modes of operations in which emission requirements are sufficiently fulfilled.
One option to operate a gas turbine engine in part load mode with reduced CO emission can be found in WO 2008/123904 A2. Pipes are present to bleed compressor air to the atmosphere, to an exhaust, or to an inlet.